In recent years, a tuning-fork type vibration gyro has been developed aiming to provide a miniaturized gyroscope. The gyro of this type typically includes two arms and a base to support the arms integrally formed of ferroelectric material. The gyro is used for detecting angular rate in a car navigation system, unintentional hand movement in a video camera, and so forth.
In FIG. 15, there is shown a schematic configuration diagram of an example of the tuning-fork type vibration gyro disclosed in the official gazette of Japanese Unexamined Patent Publication No. 2000-9476 by the present applicants. The tuning-fork type vibration gyro includes a tuning-fork type vibration body 51 constituted by two arms 52, 53, and a base 54 for supporting the arms. This tuning-fork type vibration body 51 is formed integrally with ferroelectric material of lithium tantalate (LiTaO3), lithium niobate (LiNbO3) or the like.
The bottom plane of base 54 of tuning-fork type vibration body 51 is fixed to a support substrate 56 having a slit in the center portion thereof. At this slit, support substrate 56 is connected to a support arm 57 through a bonding layer 58 formed of rubbery elastic body. Both ends of support arm 57 are bent perpendicularly to secure to a stem 55.
Stem 55 secures a circuit board 60 on which a driving circuit for vibrating arms 52, 53, a sensor circuit for detecting a signal output from tuning-fork type vibration body 51, etc. is mounted. These members are covered with a cap 59 to protect against externally applied impulse. Using such tuning-fork type vibration gyro, angular rate of rotation around z-axis, which is parallel with arms 52, 53 can be detected.
Two arms 52, 53 of tuning-fork type vibration body 51 are driven by a non-illustrated driving circuit so that each end of two arms 52, 53 vibrates in the x-axis direction. This vibration is referred to as fx mode vibration, or in-plane vibration. During this state, when tuning-fork type vibration gyro rotates around z-axis, Coriolis force is generated to two arms 52, 53 in the y-axis direction, perpendicular to x-axis, in proportion to the angular rate of rotation.
For this reason, each end of two arms 52, 53 starts fy mode vibration in the y-axis direction having magnitude proportional to the Coriolis force. The fy mode vibration is referred to as plane-vertical vibration. Coriolis force is proportional to angular rate of rotation. Therefore the angular rate of rotation can be detected by detecting the magnitude of fy mode vibration.
Next, an electrode configuration of the tuning-fork type vibration gyro is illustrated hereafter. FIG. 16A shows a perspective view of tuning-fork type vibration body 51, while FIG. 16B shows a plan view of tuning-fork type vibration body 51 viewed from the upper side.
As shown in FIG. 16B, driving electrodes 61, 62 are provided on arm 52, and driving electrodes 63, 64 are provided on arm 53. These driving electrodes are aimed to produce fx mode vibration. Also, as electrodes for detecting fy mode vibration, detecting electrodes 71, 72 and 73 are provided on arm 52, and also detecting electrodes 74, 75 and 76 are provided on arm 53.
In FIGS. 17A and 17B, a chart is shown for illustrating fx mode vibration. As shown in FIG. 17A, when driving voltage generated by an oscillator 81 is applied between driving electrodes 61 and 62, and also between electrodes 63 and 64, an electric field E is produced in arms 52, 53 to expand and contract the side faces of arms 52, 53 as a consequence of piezoelectric effect. This expansion and contraction causes fx mode vibration on the ends of arms 52, 53 in a direction shown with arrows 82, 83 in FIGS. 17A and 17B.
During this condition, when rotation around z-axis is produced as shown in FIG. 18A, Coriolis force is generated in the y-axis direction perpendicular to the vibration direction. As a result the ends of arms 52, 53 starts fy mode vibration having the magnitude proportional to the Coriolis force in the y-axis direction. Each direction of fy mode vibration is shown with arrows 84, 85.
In this case, as shown in FIG. 18B, an electric field E proportional to angular rate of rotation is produced in arms 52, 53 which are vibrating mutually in opposite directions on receiving the Coriolis force. For this reason, by detecting voltage of sensor terminals 86, 87 connected to detecting electrodes 71, 72, 73, 74, 75 and 76, angular rate of rotation can be identified.
In FIG. 19, there is shown a schematic configuration diagram of a sensor circuit for detecting the voltage proportional to fy mode vibration. This sensor circuit includes input terminals 88, 89 connected to sensor terminals 86, 87 of tuning-fork type vibration body 51; terminating resistors 21, 22 connected to input terminals 88, 89; and a differential amplifier 90 to output a signal proportional to the difference of sensor signals being input to terminals 88, 89. The sensor circuit further includes a synchronous detector 91 provided for the synchronous detection of the signal output from differential amplifier 90; an oscillator 82 for feeding a reference clock signal to synchronous detector 91; a low-pass filter (LPF) 92 having a predetermined cutoff frequency fc to cut off high frequency component included in the sensor signal; a direct-current amplifier 93 for amplifying the output of LPF 92; and output terminals 94, 95 to output detecting voltage proportional to angular rate of rotation.
As explained above, in a tuning-fork type vibration gyro, fx mode vibration is produced in arms 52, 53. Angular rate of rotation can be obtained by detecting the voltage proportional to fy mode vibration from detecting electrodes 71, 72, 73, 74, 75 and 76.
However, because tuning-fork type vibration body 51 formed of ferroelectric body is integrally configured, pyroelectric effect appears in the sensor signal as an inherent nature of ferroelectric body. This pyroelectric effect is a characteristic of electric charge generation caused by temperature change.
Namely, in the tuning-fork type vibration gyro, a superposed voltage of the following is detected as detecting voltage; a voltage generated by stress change based on the vibration; and the other voltage (pyroelectric noise) resulting from the pyroelectric effect. Accordingly, in order to detect angular rate of rotation accurately, it is necessary to reduce this pyroelectric noise as much as possible.
In FIGS. 20A and 20B, an explanatory drawing illustrating the pyroelectric noise generation mechanism is shown, as well as a conventional measure therefor. As shown in FIG. 20A(a), ferroelectric body 96 remains in a stable state at a certain temperature with spontaneous polarization P1 produced according to the current temperature. When temperature changes, different spontaneous polarization P2 is produced, to set ferroelectric body 96 to a stable state.
On the surface of ferroelectric body 96, charges corresponding to the spontaneous polarization P1, P2 are stored. Therefore, when temperature changes, the charges staying on the surface of the ferroelectric body either migrate to other ferroelectric body 96 or disappear after combined with other charges having reverse polarity, as shown in FIG. 20A(b). In this case, when charges of reverse polarity are combined abruptly, pyroelectric noise is produced resulting in signal-to-noise ratio deterioration of the tuning-fork type vibration gyro.
To cope with the above-mentioned problem, there has been known as shown in FIG. 20B that the surface of ferroelectric body 96 be covered with a high resistance film 97, formed of CrSi or the like, to suppress the pyroelectric noise generation. The reason is that high resistance film 97 enables the remainder charges on the surface of ferroelectric body 96 to be discharged gradually. Thus the pyroelectric noise generation is prevented.
However, to cover the surface of ferroelectric body 96 with high resistance film 97 requires additional process to the conventional process of manufacturing tuning-fork type vibration body 51. This may well bring about increasing production cost. In addition, because high impedance resonance of ferroelectric body 96 is used in the tuning-fork type vibration gyro, covering the surface of ferroelectric body 96 with high resistance film 97 greatly reduces detecting voltage, as well as deteriorates frequency characteristics. Furthermore, affected by surrounding humidity, the resistance value of high resistance film 97 may deviate, which degrades reproducibility.